1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly to an image reading apparatus which controls conveyance of an original in accordance with the temperature of a conveyance roller for conveying the original, and performs appropriate conveyance by determining the presence of an abnormality in the temperature detection operation.
2. Description of the Background Art
An image reading apparatus for reading an original image, which is used in an image forming apparatus such as an electrophotographic copier, a facsimile apparatus, or a printer, may be of a stationary document/moving optical system type in which the original is fixed in position and scanned by exposure light, or a moving document/stationary optical system type in which the original is moved at a predetermined speed and the conveyed original is exposed to a fixed light source. An image reading apparatus may also be of an analog type used for typical image reading apparatuses or a digital type using an imaging apparatus such as a CCD (Charged Coupled Device). In recent years, digital moving document/stationary optical system type image reading apparatuses have come into wide use due to their effectiveness in reading large originals and executing various types of image processing thereon.
A moving document/stationary optical system image reading apparatus such as that disclosed in Japanese Unexamined Patent Application Publication 2002-284388, for example, typically employs an original conveyance mechanism in which a driven roller is pressed against a conveyance roller (rubber roller), which is driven to rotate by a drive motor, such that an original is conveyed between the conveyance roller and driven roller.
The drive motor which drives the conveyance roller to rotate typically employs a stepping motor, and the length of the original is calculated from the number of steps taken during reading of the original. More specifically, in a conventional original length calculation method, the original length is usually calculated by [number of clock pulses applied to move stepping motor during reading of original×distance by which original is conveyed per clock pulse of stepping motor]=[number of clock pulses applied to move stepping motor during reading of original×distance by which original is conveyed per clock pulse (=(angle by which conveyance roller rotates per clock pulse of stepping motor/360 degrees)×(2π×radius of conveyance roller))].
However, in the background art, variation in the original conveyance speed caused by variation in the temperature of the conveyance roller is not taken into account, and improvements are required to control the original conveyance speed accurately in order to achieve an improvement in the quality of the read image. More specifically, when the conveyance roller expands or contracts in the diametrical direction in accordance with its temperature, the conveyance speed of the conveyance roller varies from a preset conveyance speed, and as a result, the read image expands or contracts in accordance with the conveyance roller temperature, leading to deterioration of the image quality. In the background art, variation in the conveyance speed caused by the temperature of the conveyance roller is not taken into consideration.
Furthermore, in the background art the length of the original is calculated from the number of steps taken by a stepping motor during reading of the original, and therefore, when the temperature of the conveyance roller that is driven to rotate by the stepping motor to convey the original varies such that the conveyance roller expands or contracts in the diametrical direction, the distance by which the original is conveyed per step of the stepping motor changes, causing an error in the original length calculation result. This leads to a magnification ratio error which becomes particularly large in the case of an elongated original.